The utility of simultaneous spatio-temporal focusing (SSTF) is gaining enthusiasm and has begun to be exploited in the field of microfluidic devices and micromachining for its ability to support large focal volumes and long working distances while mitigating nonlinear effects. Additionally, the highly localized nature of an SSTF beams' axial intensity has proved to be an optimal tool for precision tasks such as (1) the ablation of ocular tissue and localized breakdown in water with low numerical aperture beams and (2) the minimization of out-of-focus background excitation in nonlinear microscopy. A direct result of the SSTF scheme is intrinsic pulse-front tilt (PFT), a phenomenon where the arrival time of a pulse varies across the beam at focus due to spatial chirp. Optical elements inducing angular dispersion, such as a diffraction grating or prism, will result in PFT. It has been experimentally shown that PFT offers yet another degree of freedom for machining in addition to laser parameters (such as repetition rate, pulse duration, pulse energy, and wavelength), innate material properties, and the numerical aperture (NA) of the beam. More specifically, PFT gives rise to nonreciprocal writing, in which induced material modifications are dependent on the scan direction relative to the PFT.
Tuning PFT in a classical laser machining setup architecture however is cumbersome and is not a practical tool for continuous machining. Having the freedom to continuously tune the PFT in a simple manner and in a way that could be automated would broaden the capability and expand the applicability of femtosecond laser micromachining. Additionally, while the use of spatially-chirped beams has resulted in intriguing applications as described above, comparison across different groups has been complicated by the number of techniques used to create the spatial chirp (and thereby PFT). In earlier work, PFT was realized by imposing misalignment within a laser's grating compressor. (See for example, K. Osvay et al., “Angular Dispersion and Temporal Change of Femtosecond Pulses From Misaligned Pulse Compressors,” IEEE Journal of Selected Topics in Quantum Electronics 10, 213-220 (2004), the entire disclosure of which is hereby incorporated herein by reference for all that it teaches and for all purposes.) These misalignments are difficult to quantify because of the number of degrees of freedom involved. Introducing a grating angular mismatch of the gratings leads to angular spatial chirp and adjusting the retroreflection mirrors in a double-pass grating pair leads to transverse spatial chirp. A second technique in use is where a single grating is effectively imaged to the target, where the angularly dispersed frequency components cross. This configuration has applications in imaging, but is not ideal for micromachining because the spatial and temporal focal planes are typically not overlapped. That is, the Gaussian waist of the frequency dispersed beamlets and the location where the beamlets cross may not occur at the same point. SSTF may provide precise control over PFT by using a single-pass grating compressor configuration. (See, for example, Vitek, D. N., Block, E., Bellouard, Y., Adams, D. E., Backus, S., Kleinfeld, D., . . . Squier, J. A. (2010). Spatio-temporally focused femtosecond laser pulses for nonreciprocal writing in optically transparent materials. Optics Express, 18(24), 24673-24678; Vitek, D. N., Adams, D. E., Johnson, A., Tsai, P. S., Backus, S., Durfee, C. G., . . . Squier, J. A. (2010). Temporally focused femtosecond laser pulses for low numerical aperture micromachining through optically transparent materials. Optics Express, 18(17), 18086-18094; and Block, E., Greco, M., Vitek, D., Masihzadeh, O., Ammar, D. A., Kahook, M. Y., . . . Squier, J. (2013). Simultaneous spatial and temporal focusing for tissue ablation. Biomedical Optics Express, 4(6), 831-841, the entire disclosures of which are hereby incorporated herein by reference for all that they teach and for all purposes.) This setup however is inefficient and unnecessarily convoluted since a secondary SSTF grating compressor is added in conjunction with a complete chirped pulse amplification (CPA) system.